Telomerase is a specialized ribonucleoprotein (RNP) reverse transcriptase that is essential for telomere maintenance. Telomerase uses an internal RNA template to synthesize telomeric repeat sequences onto chromosome ends. Deletion of the essential RNA component of telomerase leads to progressive telomere shortening, chromosome instability and cell death in both yeast and mouse cells. This multi-subunit enzyme is regulated at several levels in human cells.
The telomerase enzyme is made up of an essential core as well as several accessory proteins. The core telomerase consists of the RNA component (Telomerase RNA, TR) and the catalytic subunit (Telomerase Reverse Transcriptase, TERT). The structure of the RNA component is conserved in ciliates, in which the RNA is 150-200 nucleotides (nt) in length. In mammalian cells, the RNA component is significantly larger, 390-450 nt. Feng, J. et al., Science, 1995, 269:1236-1241. The catalytic TERT component, first identified in the ciliate Euplotes, has homologues in yeast (EST 2), human (hTERT), and mouse (mTERT). TERT contains sequence motifs similar to reverse transcriptase and mutations of essential aspartate residues that are conserved in the catalytic triad of reverse transcriptases eliminates telomerase activity. Minimal telomerase activity can be reconstituted in an in vitro transcription/translation extract using TERT and TR components, indicating that these are sufficient for catalysis. Weinrich, S. L. et al., Nat. Genet., 1997, 17:498-502.
Both telomere length and telomerase activity have been implicated in cellular senescence and cancer. In most somatic cells, telomerase activity is not detected and telomeres shorten with each division. Allsopp, R. C. et al., Proc. Natl. Acad. Sci. USA, 1992, 89:1014-1018. Artificial elongation of telomeres by ectopic hTERT expression in primary human cells leads to telomere elongation and a bypass of cellular senescence, suggesting that telomere shortening may trigger cellular senescence in primary human cells. During immortalization of mammalian cells in culture, telomerase is activated, telomere length is stabilized, and cells continue to proliferate, suggesting that telomerase activation and telomere stabilization are required for the long term growth of cancer cells. Telomerase activity is present in the vast majority of human tumors while little activity is found in the normal tissues from which the tumors were derived. Together, these data lead to the proposal that telomerase inhibition may inhibit tumor growth. See, for example, Harley, C. B. et al., Cold Spring Harbor Symp. Quant. Biol., 1994, 59:307-315.
New evidence also indicates that telomerase plays a role in tumor initiation, not just in long term tumor growth. Telomerase null mice with significantly shortened telomeres show an increased rate of tumor formation with age compared with wild-type mice, suggesting loss of telomere function leads to increased genetic instability. Furthermore, in cells lacking the p53 gene, absence of telomerase increased the rate of focus formation after transfection of myc and RAS. This suggests that the absence of both telomerase and p53 cooperated to increase genetic instability that leads to tumor initiation. Thus, characterization of telomerase and its associated components is important to understanding tumor formation.